Interference management is an important aspect of radio resource management mechanisms in wireless communication systems. Interference may be suppressed by signal processing and/or avoided by assigning resources in an controlled manner to avoid/suppress interference. Mechanisms for interference management can be distributed to radio network nodes of the communication system, in which case information may be shared therebetween to avoid/suppress interference between the network nodes and/or between user equipment nodes (UEs) and the network nodes.
Technical Background: LTE Architecture
An example architecture of a 3G Long Term Evolution (LTE) system is shown in FIG. 1. FIG. 1 illustrates X2 logical interfaces between example eNodeBs or eNBs (Evolved Node Bs) 100′,100″,100′″ (also referred to as base stations), and S1 logical interferences between the eNBs 100′,100″,100′″ and example MMEs/S-GWs (Mobility Management Entity/Serving Gateway) 110′,110″. eNodeB is an acronym for an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) NodeB.
LTE is based on a relatively flat system architecture compared to 2G and 3G systems. Each cell is served by a respective eNB 100′,100″,100′″, and handovers between cells can be handled either via the respective MMEs 100′,100″ via the S1 interface, or directly between the eNBs via the X2 interface.
Neighbor Relations and the X2 Interface:
The cell broadcasts an identifying signature or waveform, which can be seen as a “fingerprint”, that the UEs use both as time and frequency reference, as well as to identify cells. Each waveform is identified by a Physical Cell Identity (PCI). These identifying signatures are not unique (e.g., there are 504 different PCIs in LTE), and can therefore not be used to uniquely identify a neighbor cell. In addition, each cell broadcasts as part of the system information a globally unique cell identifier (CGI).
The eNBs each maintain a neighbor relation table (NRT) for their respective cell. In general, each entry contains everything the eNB needs to know about a neighbor. Traditionally, NRTs have been populated using cell planning tools by means of coverage predictions before the installation of a eNB. Prediction errors, due to imperfections in map and building data, have necessitated that operators perform drive/walk tests to completely exhaust the coverage region and identify all handover regions. This is significantly simplified in LTE, which features the UE ANR (User Equipment Automatic Neighbor Relations) function, which means that UEs shall decode and report the CGI information of neighbor cells to the serving cell upon request. It is the CGI that the eNB uses when signaling to another eNB via the MME, since the MME routes the messages based on eNB identity which is a part of CGI.
If the policy is to establish X2 for neighbor relations and if X2 is not already available, then CGI can be used to recover the target eNB IP address (3GPP TS 36.413), which is used for X2 setup. When the X2 interface (3GPP TS 36.423) is established, the eNBs can share information about their served cells including PCIs and CGIs. Furthermore, they can share load information (3GPP TS 36.423, sections 8.3.1 and 9.1.2.1) to inform each other about the current cell load situation, for example to support interference management.
Such load information from an eNB with respect to a specific served cell may include an UL interference overload indication to inform about the interference situation at the eNB receiver with respect to a served cell and for different frequency resources, in particular to inform about high uplink interference.
The load information may also include an uplink high interference indication to inform about potentially high induced interference at a target cell and at indicated frequency resources. This is typically used when a UE served by a first cell reports a second cell as a candidate cell to inform the eNB serving the second cell that it may experience rather high uplink interference at the frequency resources where the UE is allocated.
Random Access
The random access (RA) serves as an uplink control procedure to enable the UE to access the network. The RA procedure serves two main purposes. First, the RA procedure lets the UE align its UL timing to that expected by the eNB in order to minimize interfering with other UEs transmissions. UL time alignment is a requirement in E-UTRAN before data transmissions can commence. Second, the RA procedure provides an ability for the UE to notify the network of its presence and enables the eNB to give the UE initial access to the system. In addition to the usage during initial access, the RA will also be used when the UE has lost the uplink synchronization or when the UE is in an idle or a low-power mode.
FIG. 2 illustrates the primary four operation/method steps/phases of a RA procedure. Step 1 includes transmitting a random access preamble from a UE 200 to a eNB 100, which enables the eNB 100 to estimate the transmission timing of the UE 200. Uplink synchronization is necessary as the UE otherwise cannot transmit any uplink data.
To generate the random access preamble, the UE 200 obtains information about which preambles are available (either to select one at random or to use a specified one), whether one or repeated preambles should be used, what the desired received power level should be at the eNB 100, what power increase step that should be used in case of failed preamble reception, what the maximum number of random access preamble transmission is, when it is allowed to transmit the preamble, etc.
When the UE 200 obtains the Phase I information via dedicated signaling, such as random access as part of handover (the dedicated signaling originated from the target cell, forwarded to the UE 200 by the serving cell), a specific preamble may be configured. In addition, a designated timer T304 is started with a value provided by the dedicated signaling.
Step 2 (FIG. 2) includes the eNB 100 transmitting a random access response, such as a timing advance command to the UE 200 to cause correction of the uplink timing, based on the timing of arrival measurement in Step 1. In addition to establishing uplink synchronization, the Step 2 also assigns uplink resources and temporary identifier to the UE 200 to be used in a subsequent Step 3 in the RA procedure.
The UE 200 monitors a Packet Data Control CHannel (PDCCH) of the cell for random access response in the RA response window, which starts at the subframe that contains the end of the preamble transmission plus three subframes and has the length ra-ResponseWindowSize.
If no response has been received, and the max number of preamble transmissions has been reached, or the timer T304 has expired, the handover attempt is considered failed and higher layer is informed. Then, the UE 200 initiates the RRC connection reestablishment procedure to restore the connection to the source cell, specifying the reestablishment cause to handover failure. Furthermore, a radio link failure report is prepared.
Step 3 (FIG. 2) includes the UE 200 transmitting message signalling to the eNB 100. A primary function of this message is to uniquely identify the UE 200. The exact content of this signalling depends on the state of the UE 200, e.g., whether it is previously known to the eNB 100 or not.
Step 4 (FIG. 2) includes contention resolution in case multiple UEs 200 have attempted to access the system on the same resource. In case of handover, the target eNB 100″ may signal random access information to the source eNB 100′, which will further convey that information to the UE 200. This information may comprise a reserved RA preamble for unique identification already at Phase 2. This is known as contention free random access.
Channel Sounding
Sounding resources are used to transmit reference symbols over the entire or parts of the uplink bandwidth. By transmitting uplink sounding reference signals, the UE 200 can provide the eNB 100 with information about uplink channel quality. This information can be utilized e.g. for uplink channel dependent scheduling, uplink link adaptation, and also for downlink beam forming in case of reciprocal channels as in time division duplex (TDD). The sounding reference signals are transmitted on the last symbol in the subframe. Several UEs 200 can transmit sounding in the same subframe. This is made possible by assigning a set of sounding reference signals to each cell, preferably such that these signals are locally unique in the sense that no other cell in the vicinity is assigned the same sounding reference signals.
Multiple Carriers and Carrier Aggregation
In 3GPP LTE, a cell is associated with a downlink and optionally an uplink carrier, as well as a coverage area. If the operator has license for more than one LTE carrier, then the eNB 100 can configure multiple cells, each assigned a different carrier. Then, the eNB 100 may (re)allocate UEs 200 to different carriers depending on their service needs and the available capacity per carrier in the network. The allocation mechanism is essentially the intra-site handover mechanism.
An alternative is carrier aggregation, where the UE 200 is assigned a primary component carrier and zero or more secondary component carriers to enable service over wide bandwidths. It is thereby possible to (re)assign the frequency resources over the available component carriers in the scheduler.
FIGS. 3A-3B illustrate example frequency resource assignment operations and methods. Referring to FIG. 3A, the UE 200 is assigned frequency resources one carrier at a time out a plurality of available carriers. Referring to FIG. 3B, a plurality of carriers are aggregation for simultaneous use by the UE 200, were the frequency resource allocation can span all available carriers.
Network Management Architecture
In addition to the user and control planes specified in 3GPP, there is architecture for network management to support configuration, equipment management, fault management, performance management, etc.
FIG. 4 illustrates a block diagram of an example management system. The node elements (NE) 402, also referred to as eNBs, are managed by a domain manager (DM) 400′,400″, also referred to as the operation and support system (OSS). Sometimes the individual elements (eNBs) 100′,100″ are considered handled by an element manager (EM), which is a part of the DM 400′,400″. Typically, a DM manages only equipment from the same vendor. The DM 400′,400″ tasks include configurations of the network elements, fault management and performance monitoring. The latter can mean that extensive data from events and counters is regularly transferred from the network elements up to the DM 400′,400″.
The DM 400′,400″ may further be managed by a network manager (NM) 402 via Itf-N. Two NEs 100′,100″ are interfaced by X2, whereas the interface between two DMs 400′,400″ is referred to as Itf-P2P. This means that multi-vendor management can be handled either via the common NM 402 and the interface Itf-N, or via the peer-to-peer interface Itf-P2P. Furthermore, the X2 interface between eNBs 100′,100″ also supports some management. Moreover, this interface is standardized and therefore works between eNBs from different vendors.
Heterogeneous Networks
In a cellular network there will often be areas with high traffic, i.e. high concentration of users. In those areas it would be desirable to deploy additional capacity to ensure user satisfaction. The added capacity could then be in the form of additional high power (macro) base station or to deploy nodes with lower output power and thus covering a smaller area in order to concentrate the capacity boost on a smaller area
There will also be areas with bad coverage where there is a need for coverage extension, and again one way to do that is to deploy a node with low output power to concentrate the coverage boost in a small area.
One argument for choosing nodes with lower output power in the above cases is that the impact on the high power (macro) network can be minimized, e.g. it is a smaller area where the high power (macro) network may experience interference.
Currently there is a strong drive in the industry in the direction towards the use of low power nodes. The different terms used for this type of network deployments are Heterogeneous networks, multilayer networks or shortly HetNets.
FIG. 5 illustrates a high power (macro) base station (the illustrated high tower) which provides a wide area coverage (also called macro cell). It also shows low power nodes that are deployed to provide small area capacity/coverage. In this example pico base stations, relays and home base stations (femto cells) are shown. Although the figure shows clusters of femto cells, single cell deployments may also exist.
When a UE served by a high power (macro) base station is closely spaced to a femto base station, possibly with restricted access, then the femto base station may induce significant downlink interference to the UE. One mechanism to avoid this is via two carriers f1 and f2 available at the high power (macro) base station, and only carrier f2 available at the femto based station as in FIG. 6. UEs served by the high power (macro) base station at carrier f2 and interfered in the downlink at carrier f2 are handed over to carrier f1. The carrier f1 is sometimes referred to as an escape carrier.
In a similar solution, UEs served by the high power (macro) base station can aggregate the primary component carrier f1 with the secondary component carrier f2 if the latter is not interfered by the femto base station in which case the UE operates only on the primary component carrier f1. Nevertheless, the secondary carrier allocated to the UE could be free from interference from the femto base station, but its use could cause high interference to the femto base station cell.
Known approaches to overcoming the above disadvantages suffer from several drawbacks. Specifically, cell and carrier selection is based on downlink reference signal measurements by the UE. This means that the assigned carriers consist of the most appropriate choice (from a radio efficiency point of view) relevant for the downlink, but not necessarily for the uplink. Furthermore, detailed uplink interference coordination via the UL high interference indication is driven by the knowledge that a particular UE may interfere with a specific second cell. However, it is not always possible to identify this interference relation based on downlink measurements. In heterogeneous networks, it is possible that a UE served by a high power (macro) eNB may induce significant interference to a low power base station even without detecting the downlink from the low power base station.
The approaches and presently recognized problems described above in this section could be pursued, but are not necessarily approaches and/or problems that have been previously conceived or pursued. Therefore, unless otherwise clearly indicated herein, the approaches and problems described above in this section are not prior art to claims in any application claiming priority from this application and are not admitted to be prior art by inclusion in this section.